asserta/retract via C++

[aLu]

Hello,

I have integrated GNU-Prolog in a C++ projekt for some automated planning, but
now I'm having some trouble updating my predicates containing dynamic 
knowledge. 

A Pl_Query_Call with asserta or retract (on dynamic predicates) does not seem to
work in my project executable. (Simple calls do work)

Still it works in an executable made of .pl files only. 
Is it even possible to manipulate prolog predicates via C++? How do I have to
implement those instructions?

Thanks in advance